An automatic transmission establishes various power flow paths having different speed ratios by selectively engaging and disengaging a number of shift elements. The shift elements include friction clutches. The torque capacity of each friction clutch is determined based on a signal from a transmission controller. For example, a controller may vary the pressure of hydraulic fluid in a particular passageway, called a circuit. Fluid from the circuit may be routed to a piston apply chamber of a clutch causing the piston to exert a force related to the fluid pressure. The piston force may squeeze friction plates between separator plates to establish a torque capacity of a clutch.
Hydraulic fluid may be supplied to a transmission valve body line pressure circuit by an engine driven pump, an electric pump, an accumulator, or other means. The pressure is often regulated to a line pressure by a regulator valve or by adjusting the displacement of the pump. Various circuits in the valve body are supplied with fluid at controlled pressures less than line pressure using pressure control valves.
Sometimes, a latch valve is interposed between a pressure control valve and a corresponding clutch apply circuit. The latch valve creates a non-linear relationship between clutch apply pressure and the controlled pressure. When the controlled pressure is less than a latch pressure, the latch valve directs flow from the controlled pressure circuit to the clutch apply circuit. This is called the de-latched state. When the controlled pressure exceeds the latch pressure, the latch valve directs flow from the line pressure circuit to the clutch apply circuit. This is called the latched state. The latch valve permits a low clutch gain for more accurate control of clutch apply pressure in the unlatched state while still allowing full line pressure in the latched state.
When the transmission is in a steady speed ratio, the clutch apply circuits for the engaged clutches are each supplied with a high enough pressure that the clutch torque capacity exceeds the torque transmitted by the clutch. This may be accomplished by commanding a pressure high enough to enter the latched state. When the controller determines that a speed ratio change is needed, the controller executes a shift by releasing one shift element called an off-going element and engaging another shift element called an on-coming element. For the shift to feel smooth to vehicle occupants, it is important that the torque capacity of the on-coming element and the torque capacity of the off-going element be coordinated with respect to one another and with respect to the engine torque. For example, if the off-going clutch is released prematurely, then the output torque will drop excessively and the engine speed will rise. This phenomenon is called an engine flare. On the other hand, if the off-going element is released too late, then the two shift elements will resist one another and the output torque will drop excessively. This phenomenon is called a tie-up. To actively control the torque capacity of the off-going element, the latch valve must be in the de-latched state. If the latch valve for the off-going element was previously in the latched state, then a state transition must occur before the controlled can execute the shift.